Generally, the mixer is a frequency conversion element used in a radio transceiver. FIG. 1 depicts a block diagram of a conventional mixer. Basically, a mixer comprises a transconductor 10, a switch quad 20, and a load circuit 30. The load circuit 30 comprises a first load (l1) and a second load (l2). Each of the first load (l1) and the second load (l2) has a first terminal to which a voltage source (Vcc) is applied, and a second terminal serving as an output terminal (Out).
The switch quad 20 comprises four n-channel transistors Mn13, Mn14, Mn15, and Mn16. The drains of the transistors Mn13, Mn15 are coupled to the second terminal of the first load (l1), and the drains of the n-channel transistors Mn14, Mn16 are coupled to the second terminal of the second load (l2). Moreover, the gates of the n-channel transistors Mn13, Mn16 are coupled to each other, and the gates of the n-channel transistors Mn14, Mn15 are coupled to each other. An LO signal (Local Oscillator Signal) is applied to the gates of the n-channel transistors Mn13 and Mn14. Moreover, the sources of the n-channel transistors Mn13 and Mn14 are coupled to each other to provide a first current path of the switch quad 20. The sources of the n-channel transistors Mn15 and Mn16 are coupled to each other to provide a second current path of the switch quad 20.
The transconductor 10 comprises two n-channel transistors Mn17 and Mn18. The drain of the n-channel transistor Mn17 is coupled to the first current path of the switch quad 20, and the drain of the n-channel transistor Mn18 is coupled to the second current path of the switch quad 20. Voltage signals Vin+ and Vin− are applied to the gates of the n-channel transistors Mn17 and Mn18, respectively. Moreover, the sources of the n-channel transistors Mn17 and Mn18 are coupled to the drain of the n-channel transistor Mn19. The source of the n-channel transistor Mn19 is grounded. The n-channel transistor Mn19 serves as a constant current source due to the gate of the n-channel transistors Mn19 being applied thereto a fixed DC voltage.
FIG. 2 is a timing diagram showing the input/output signals of the conventional mixer. In a small signal differential model, the input voltage signal (Vin=Vin+−Vin−) is converted to a current signal (Iin) by the transconductor 10. When the current signal (Iin) is flowing through the first current path and the second current path of the switch quad 20, the current signal (Iin) is converted to a frequency converted current signal by the driving of the LO signal. Then, the frequency converted current signal is converted to a voltage signal by the load circuit 30 and the voltage signal is outputted from the output terminal (Out).
FIG. 3 is a diagram showing a typical voltage-current transfer function of the transconductor in the conventional mixer. As depicted in FIG. 3, the voltage to current transfer function is not linear, but quadratic. Because of the non-linear characteristic of the n-channel transistors Mn17 and Mn18, the transfer function between the voltage and the current in the transconductor 10 is non-linear. The conventional mixer suffers from the non-linear characteristic of the transconductor 10, which prohibits this type of mixer in the high linearity application such as wireless local area network (WLAN) or code division multiple access (CDMA) transmitters.
There are several conventional mixers employing transconductors with linear transfer functions. FIG. 4 depicts a mixer, disclosed in IEEE Journal of Solid-State Circuits. Vol. 40, No. 5, May 2005, having a transconductor with a linear voltage-current transfer function. The following description focuses on the circuit design of the transconductor.
As depicted in FIG. 4, the transconductor 40 comprises two n-channel transistors Mn20 and Mn21, two operational amplifiers OP3 and OP4, a resistor R4, and two current sources I4th and I5th. The drains of the n-channel transistors Mn20 and Mn21 are coupled to the first current path and the second current path of the switch quad, respectively. The output terminal of the operational amplifier OP3 is coupled to the game of the n-channel transistor Mn20, and the negative input terminal of the operational amplifier OP3 is coupled to the source of the n-channel transistor Mn20. The output terminal of the operational amplifier OP4 is coupled to the gate of the n-channel transistor Mn21, and the negative input terminal of the operational amplifier OP4 is coupled to the source of the n-channel transistor Mn21. The input voltage signals Vin+ and Vin− are applied to the positive input terminals of the operational amplifiers OP3 and OP4, respectively. Moreover, the current source I4th is coupled between the source of the n-channel transistor Mn20 and the ground. The current source I5th is coupled between the source of the n-channel transistor Mn21 and the ground. The resistor R4 is coupled between the source of the n-channel transistor Mn20 and the source of the n-channel transistor Mn21.
In the operational amplifier OP3, due to its high open loop gain, the voltage of the positive input terminal is equal to the voltage of the negative input terminal. Similarly, the voltage of the positive input terminal is equal to the voltage of the negative input terminal in the operational amplifier OP4. Therefore, in the small signal model, the current Iin is given by the equation Iin=(Vin+−Vin−)/R4. The linear voltage-current transfer function can be achieved in the transconductor 40 shown in FIG. 4.
FIG. 5 depicts a mixer, which is disclosed in IEEE Journal of Solid-State Circuits, Vol. 38, No. 12, December 2003, having a transconductor with a linear voltage-current transfer function. The transconductor 50 comprises two p-channel transistors Mp1 and Mp2, two operational amplifiers OP5 and OP6, a resistor R5, and four current sources I6th, I7th, I8th, and I9th. The drains of the p-channel transistors Mp1 and Mp2 are coupled to the first current path and the second current path of the switch quad, respectively. The output terminal of the operational amplifier OP5 is coupled to the gate of the p-channel transistor Mp1, and the negative input terminal of the operational amplifier OP5 is coupled to the source of the p-channel transistor Mp1. The output terminal of the operational amplifier OP6 is coupled to the gate of the p-channel transistor Mp2, and the negative input terminal of the operational amplifier OP6 is coupled to the source of the p-channel transistor Mp2. The voltage signals Vin+ and Vin− are applied to the positive input terminals of the operational amplifiers OP5 and OP6, respectively. Moreover, the current source I6th is coupled between the source of the p-channel transistor Mp1 and the voltage source (Vcc), the current source I7th is coupled between the source of the of the p-channel transistor Mp2 and the voltage source (Vcc), the current source I8th is coupled between the drain of the p-channel transistor Mp1 and the ground, and the current source I9th is coupled between the drain of the p-channel transistor Mp2 and the ground. The resistor R5 is coupled between the sources of the p-channel transistor Mp1 and the p-channel transistor Mp2.
Similarly, in the operational amplifier OP5, due to its high open loop gain, the voltage of the positive input terminal is equal to the voltage of the negative input terminal, so that the voltage of the positive input terminal is equal to the voltage of the negative input terminal in the operational amplifier OP6. Therefore, the current Iin is given by the equation Iin=(Vin+−Vin−)/R5 in the small model. The linear voltage-current transfer function can be achieved in the transconductor 50 shown in FIG. 5.
FIG. 6 depicts a mixer, disclosed in IEEE Journal of Solid-State Circuits, Vol. 39, No. 8, August 2004, having a transconductor with a linear voltage-current transfer function. The transconductor 60 comprises two n-channel transistors Mn22 and Mn23, a differential operational amplifier 63, two resistors R6 and R7, a compensating circuit 64 and a compensating circuit 66. The drains of the n-channel transistors Mn22 and Mn23 are coupled to the first current path and the second current path of the switch quad, respectively. The first output terminal of the differential operational amplifier 63 is coupled to the gate of the n-channel transistor Mn22, and the negative input terminal of the differential operational amplifier 63 is coupled to the source of the n-channel transistor Mn22. The second output terminal of the differential operational amplifier 63 is coupled to the gate of the n-channel transistor Mn23, and the positive input terminal of the differential operational amplifier 63 is coupled to the source of the n-channel transistor Mn23. The compensating circuit 64 is coupled between the first output terminal of the differential operational amplifier 63 and the ground, and the compensating circuit 66 is coupled between the second output terminal of the differential operational amplifier 63 and the ground, wherein both the compensating circuit 64 and the compensating circuit 66 comprise a capacitor and a resistor coupled in series. The first terminal of the resistor R6 is coupled to the negative input terminal of the differential operational amplifier 63, and the first terminal of the resistor R7 is coupled to the positive input terminal of the differential operational amplifier 63. The voltage signals Vin+ and Vin− are applied to the second terminal of the resistor R6 and the second terminal of the resistor R7, respectively.
In view of the differential operational amplifier 63, due to its high open loop gain, the voltage of the positive input terminal is equal to the voltage of the negative input terminal. Therefore, the current Iin is given by the equation Iin=(Vin+−Vin−)/(R6+R7) in the small signal differential model. The linear voltage-current transfer function can be achieved in the transconductor 60 shown in FIG. 6.
FIG. 7 depicts a mixer, which is disclosed in IEEE Journal of Solid-State Circuits, Vol. 41, No. 8, August 2006, having a transductor with a linear voltage-current transfer function. The transconductor 70 comprises four n-channel transistors Mn24, Mn25, Mn26, and Mn27, four current sources I10th, I11th, I12th, and I13th, a resistor R8, and two p-channel transistors Mp3 and Mp4. The drains of the n-channel transistors Mn24 and Mn25 are coupled to the first current path and the second current path of the switch quad, respectively. The current source I12th is coupled between the source and the gate of the n-channel transistor Mn24, and also coupled between the source and the gate of the n-channel transistor Mn26. The current source I13th is coupled between the source and the gate of the n-channel transistor Mn25, and also coupled between the source and the gate of the n-channel transistor Mn27. The drain of the p-channel transistor Mp3 is coupled to the gate of the n-channel transistor Mn26, and the source of the p-channel transistor Mp3 is coupled to the drain of the n-channel transistor Mn26. The drain of the p-channel transistor Mp4 is coupled to the gate of the n-channel transistor Mn27, and the source of the p-channel transistor Mp4 is coupled to the drain of the n-channel transistor Mn27. The resistor R8 is coupled between the source of the p-channel transistor Mp3 and the source of the p-channel transistor Mp4. The current source I10th is coupled between the source of the p-channel transistor Mp3 and the voltage source (Vcc), and the current source I11th is coupled between the source of the p-channel transistor Mp4 and the voltage source (Vcc).
In view of the p-channel transistors Mp3 and Mp4, the voltage of the gate is equal to the voltage of the source due to each transistor being connected as a super source follower. Therefore, the current I′ is given by the equation I′=(Vin+−Vin−)/R8 in the small signal model. The current Iin is given by the equation Iin=NI′=N(Vin+−Vin−)/R8 if the aspect ratio of Mn24 to Mn26 is N:1 and the aspect ratio of Mn25 to Mn27 is N:1. The linear voltage-current transfer function can be achieved in the transconductor 70 shown in FIG. 7.
FIG. 8 depicts a mixer, which is disclosed in IEEE Journal of Solid-State Circuits, Vol. 41, No. 5, May 2006, having a conductor with a linear voltage-current transfer function. The transconductor 80 comprises four n-channel transistors Mn5, Mn6, Mn7, and Mn8, a buffer 87, and two resistors R9 and R10. The drains of the n-channel transistors Mn5 and Mn6 are coupled to the first current path and the second current path of the switch quad, respectively. The sources of the n-channel transistors Mn5, Mn6, Mn7, and Mn8 are grounded. The gate and the drain of the n-channel transistor Mn7 are coupled together, the gate of the n-channel transistor Mn5 and the gate of the n-channel transistor Mn7 are coupled together, and thus the n-channel transistors Mn5 and Mn7 function as a current mirror. The gate and the drain of the n-channel transistor Mn8 are coupled together, the gate of the n-channel transistor Mn6 and the gate of the n-channel transistor Mn8 are coupled together, and thus the n-channel transistors Mn6 and Mn8 form a current mirror. The resistor R9 is coupled between the drain of the n-channel transistor Mn7 and a voltage source (Vcc), and the resistor R10 is coupled between the drain of the n-channel transistor Mn8 and the voltage source (Vcc). The voltage signals Vin+ and Vin− are applied to the two input terminals of the buffer 87, and the two output terminals of the buffer 87 are coupled to the gates of the n-channel transistors Mn7 and Mn8, respectively.
In the small signal model, the current I′ is given by the equation I′=(Vin+−Vin−)/(R9+R10). Also, the current Iin is given by the equation Iin=NI′=N (Vin+−Vin−)/(R9+R10) if the aspect ratio of Mn5 to Mn 7 is N:1 and the aspect ratio of Mn6 to M8 is N:1. The linear voltage-current transfer function can be achieved in the transconductor 80 shown in FIG. 8.
To achieve the transconductor with a linear voltage-current transfer function, the transconductors in the mixers shown in FIGS. 4, 5, and 6 utilize negative feedback at the source terminals of the transistors. The purpose of the present invention provides a novel mixer having a linear voltage-current transfer function.